Open Pixel-art Platformer
github.com
github.com
Looks like the key points are "fairly modern engine with full hardware acceleration", "it runs on darned near everything", and "a bunch of pixel-art specific features in it; we wanted to be able to do a lot of the typical pixel-art tricks you're familiar with from classic games".
Why should you be excited?
Well, the engine supports nearly every trick Bret Victor has shown in his Inventing on Principle talk. While looking really old-school it's actually rather advanced modern engine with a lot of love and advanced computer science put in it. For example it's scripting engine uses pure functional programming disguised to look like something imperative.
https://github.com/anura-engine/anura
(see http://www.frogatto.com/developer for details)